In the past, metal sheets have often been painted after being worked in order to improve the rust preventing performance and appearance. For example, steel sheet is often given a baked on paint after working. Postcoating metal sheet after working necessitates a large space for the painting work and is not necessarily good in terms of the work environment either. Further, disposal of the waste products generated there is necessary.
Explaining this with reference to the case of steel sheet, in recent years there has been much activity in use of precoated steel sheet painted in advance so as to eliminate the painting process after working and thereby lower the costs. In precoated steel sheet, the base steel sheet is conversion-treated, then formed with a primer layer and then formed with one or more coat layers. The primer layer is sometimes omitted. The back surface is often painted with a single layer called a “service coat”.
In the production of conventional precoated steel sheet having two or more coat layers in addition to the primer layer, the practice has been to form a single lower layer paint film on the primer layer using a curtain coater or roll coater, dry and bake it on, then again use a curtain coater or roll coater to form a single upper layer paint coat and dry and bake it on.
As the advantages of the precoated steel sheet produced in the past, two points may be mentioned: (1) the paintin work is performed by the manufacturer and there is no painting work at the user, so the solvent can be disposed of all together and (2) an entire lot can be painted together, so the quality is stable.
On the other hand, the following may be mentioned as difficulties with conventional precoated steel sheet: (1) When producing the sheet by the method of coating and drying single layers at a time, since the sheets are worked after painting, the coats sometimes peel off at the time of working. (2) From the viewpoint of production costs, coating and drying two layers repeatedly was suitable (painting lines for giving three or more layers were rare), so if desiring to coat three or more layers of coats so as to improve the performance of the coats, the sheets had to be passed through a two-layer coat line two times and therefore the productivity fell.
If it were possible to apply simultaneous multilayer coating at the time of production of precoated steel sheet, the following advantages could be anticipated. First, two or more layers of coats could be simultaneously painted, so the process could be simplified compared with the conventional repeated painting of single layers. Second, three or more layer multilayer coats could also be coated simultaneously, so a further improvement in efficiency could be expected. Third, in the past, in “wet-on-wet painting” forming another paint layer on a not yet dried paint layer and simultaneously drying and baking them on, the adhesion between the coats is reportedly improved, so the adhesion between coats can be expected to be improved over that of the conventional repeated painting of single layers.
In the past, several proposals have been made for multilayer simultaneous coating of the surface of steel sheet. For example, JP 6-190335 A describes that if making the surface tension of a lower layer higher than the surface tension of an upper layer in painted steel sheet produced by a multilayer simultaneous coater, the sharpness is improved. The Examples describe that “as the lower layer paint, a polyester based primer paint was used, while as the upper layer paint, a high molecular weight polyester based top coat paint was used. The paints were adjusted in surface tension by surfactants to make the surface tension of the lower layer paint 35 mN/m and make the surface tension of the upper layer paint 28 mN/m. These paints were simultaneously coated by a slide hopper type curtain coater so that the dried thickness of the primer coat became 5 μm and the top coat became 10 μm. The steel sheet coated with the paints was then dried and cured by an induction heating oven, whereupon a high sharpness steel sheet could be obtained.”
JP 6-190336 A proposes a method of production of a patterned steel sheet. The Examples describe use of high molecular weight polyester based top coat paints as paints, making the paint forming the upper layer on the surface of the steel sheet white, making the paint forming the lower layer black, adjusting the surface tensions of the paints by surfactants so as to create patterns and making the surface tension of the upper layer paint 35 mN/m and the surface tension of the lower layer paint 28 mN/m, and simultaneously coating these two paints by a slide hopper type coater to give dried thicknesses of 10 μm each.
Production of a precoated metal sheet by multilayer simultaneous coating using a slide hopper type coater is described in for example JP 6-190334 A etc.
One type of precoated metal sheet, that is, top clear type precoated steel sheet, combines a topmost layer clear paint and a lower layer colored base coat paint so as to give a heavy feel and luxurious feel to the appearance. This has been used for automobiles, home electrical appliances (in particular outdoor electrical appliances), building materials (in particular outdoor building materials), etc. In general, in such top clear type precoated steel sheet, as the topmost layer acrylic paint, a polyester-based, acryl-based, PET-based, silicone polyester-based, silicone-based, silicone acryl-based, fluorine-based, or other resin has been used.
In the case of outdoor use, mainly a polyester-based, acryl-based, fluorine-based, silicone-based, silicone polyester-based, or silicone acryl-based clear paint has been used. However, a polyester-based one is inferior in weather resistance, so has the problem that with long term use, the clear paint deteriorates and peels off, while an acryl-based one is superior to a polyester-based one in weather resistance, but is still insufficient in weather resistance. On the other hand, as an example of use of a fluorine-based one, for example, JP 4-131165 A reports use of a water-based fluorine resin for a top clear paint. JP 10-5693 A reports the method of use of a fluorine resin for a top clear paint of a cosmetic metal sheet having an appearance close to that of natural stone or rock. While it is true that use of a fluorine-based resin for a clear paint in this way improves the weather resistance, there was the problem that the cost became higher.
In the case of a top clear type precoated metal sheet of a type comprised of a plurality of coat layers in this way in which the topmost layer is a transparent or semitransparent clear layer, there is the feature that even if the topmost layer deteriorates, there will be little chalking due to exposure of the pigment and therefore the appearance will remain superior at the time of long term use.
However, there was the problem that the interface between the clear layer and the lower layer coat below it deteriorated due to light and therefore the lower layer coats and clear layer easily peeled apart. The reason is believed to be that the major cause of the deterioration, UV rays, pass through the clear layer and cause the surfacemost layer of the lower layer coats to deteriorate. To prevent this deterioration, the methods have been adopted of adding an expensive UV absorbent to the clear layer (Journal of the Japan Coating Technology Association, 2001, vol. 36, no. 8, pp. 266 to 271) etc., but in general this is costly and is insufficient in effect of preventing deterioration.
Precoated metal sheet is shaped at the demand side, so superior workability is considered of the primary importance, but in addition to this, in particular corrosion resistance is sought in some cases. For example, JP 8-309917 A discloses precoated metal sheet superior in not only workability, but also end face corrosion resistance. In the precoated metal sheet described in JP 8-309917 A, the two sides of galvanized metal sheet are formed with coats containing a rust preventing pigment in an amount of 25% or more. It describes that if the content of the rust preventing pigment becomes 50%, the workability sometimes falls. Further, it describes that a topcoat can be formed. The Examples describe an example of production by coating a paint containing a rust preventing pigment by a roll coater, then baking it, followed by coating a topcoat paint by a roll coater again and then baking it, that is, two coatings and two bakings (in general, abbreviated as the “2 coat, 2 bake (2C2B)”).
An ordinary precoated metal sheet is produced by separately forming a lower layer coat and an upper layer coat by the above explained method of two coatings and two bakings. The method of forming two layers of paint film (comprised of a colored base paint layer and a clear paint layer) by the wet-on-wet method (that is, forming a clear paint layer over a colored base paint layer before baking) and baking them once to form two layers of coats (in general, abbreviated as the “2 coat, 1 bake (2C1B)”) is also known (for example, JP 11-19583 A).
As described in JP 8-309917 A, it is known that if increasing the content of the rust preventing pigment in the coats, the precoated metal sheet is impaired in workability. Further, in a general precoated metal sheet of a two-layer structure of a lower layer and upper layer, it is known that it is no longer possible to raise the corrosion resistance of the precoated metal sheet as desired as will be understood from the fact that if increasing the rust preventing pigment of the lower layer, saltwater invades the interface of the two layers in a saltwater spray test (SST) and causes blisters.